U.S. patent application number 15/398101 was filed with the patent office on 2017-04-27 for electrostatic trap.
The applicant listed for this patent is Thermo Fisher Scientific (Bremen) GmbH. Invention is credited to Wilko BALSCHUN, Eduard V. DENISOV, Stevan R. HORNING, Gerhard JUNG, Alexander A. MAKAROV.
Application Number | 20170117130 15/398101 |
Document ID | / |
Family ID | 34835118 |
Filed Date | 2017-04-27 |
United States Patent
Application |
20170117130 |
Kind Code |
A1 |
MAKAROV; Alexander A. ; et
al. |
April 27, 2017 |
Electrostatic Trap
Abstract
An electrostatic trap such as an orbitrap is disclosed, with an
electrode structure. An electrostatic trapping field of the form
U'(r, .PHI., z) is generated to trap ions within the trap so that
they undergo isochronous oscillations. The trapping field U'(r,
.PHI., z) is the result of a perturbation W to an ideal field U(r,
.PHI., z) which, for example, is hyperlogarithmic in the case of an
orbitrap. The perturbation W may be introduced in various ways,
such as by distorting the geometry of the trap so that it no longer
follows an equipotential of the ideal field U(r, .PHI., z), or by
adding a distortion field (either electric or magnetic). The
magnitude of the perturbation is such that at least some of the
trapped ions have an absolute phase spread of more than zero but
less than 2 .pi. radians over an ion detection period T.sub.m.
Inventors: |
MAKAROV; Alexander A.;
(Bremen, DE) ; DENISOV; Eduard V.; (Bremen,
DE) ; JUNG; Gerhard; (Delmenhorst, DE) ;
BALSCHUN; Wilko; (Bremen, DE) ; HORNING; Stevan
R.; (Delmenhorst, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Thermo Fisher Scientific (Bremen) GmbH |
Bremen |
|
DE |
|
|
Family ID: |
34835118 |
Appl. No.: |
15/398101 |
Filed: |
January 4, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14832978 |
Aug 21, 2015 |
9570283 |
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15398101 |
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14596187 |
Jan 13, 2015 |
9117647 |
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14832978 |
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14269452 |
May 5, 2014 |
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14596187 |
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13737771 |
Jan 9, 2013 |
8716654 |
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14269452 |
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13474020 |
May 17, 2012 |
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13737771 |
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12749334 |
Mar 29, 2010 |
8198581 |
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13474020 |
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10587478 |
Sep 4, 2008 |
7714283 |
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PCT/GB2006/002028 |
Jun 5, 2006 |
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12749334 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01J 49/02 20130101;
H01J 49/406 20130101; H01J 49/425 20130101; H01J 49/0027 20130101;
H01J 49/4245 20130101; H01J 49/282 20130101 |
International
Class: |
H01J 49/42 20060101
H01J049/42; H01J 49/28 20060101 H01J049/28; H01J 49/40 20060101
H01J049/40; H01J 49/00 20060101 H01J049/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 3, 2005 |
GB |
0511375.8 |
Claims
1. A method of analyzing ions on mass-to-charge ratio m/q in a
multi-reflecting system having an electrode assembly, comprising:
a. applying a substantially electrostatic potential to at least a
part of the electrode assembly, so as to provide multiple
isochronous reflections and/or deflections of the ions in a volume
V; b. applying a time-dependent perturbation of potential during a
time of motion T of the ions to change rate of increase of phase
spread of the ions.
2. The method of claim 1 wherein the rate of increase of phase
spread of the ions is substantially higher or lower at a beginning
of the time of motion T than an average of the rate of increase of
phase spread of the ions over the time of motion T.
3. The method of claim 2 wherein the rate of increase of phase
spread of the ions is substantially higher at the beginning of the
time of motion T in order to increase a size of an ion packet of
the ions and reduce space-charge effects.
4. The method of claim 1 wherein the multi-reflecting system
includes a pulsed ion injection device.
5. The method of claim 4 further comprising pulsing ions from the
injection device towards the electrode assembly.
6. The method of claim 4 further comprising adjusting potentials on
parts of the electrode assembly and/or injection device during
injections in order to affect the rate of increase of phase spread
of the ions.
7. The method of claim 1 wherein electrodes of the electrode
assembly are one of: parts of lens stack, deflector, and
field-defining electrodes.
8. The method of claim 1 wherein the multi-reflecting system is one
of: a) electrostatic trap; b) multi-reflection time-of-flight mass
spectrometer; and c) multi-deflection time-of-flight mass
spectrometer.
9. The method of claim 1 wherein the time-dependent perturbation is
a perturbation of an ideal field.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of pending U.S.
patent application Ser. No. 14/832,978 filed Aug. 21, 2015, which
is a continuation of U.S. patent application Ser. No. 14/596,187
filed Jan. 13, 2015, now U.S. Pat. No. 9,117,647, which is a
continuation of U.S. patent application Ser. No. 14/269,452 filed
May 5, 2014, now abandoned, which is a continuation of U.S. patent
application Ser. No. 13/737,771 filed Jan. 9, 2013, now U.S. Pat.
No. 8,716,654, which is a continuation of U.S. patent application
Ser. No. 13/474,020 filed May 17, 2012, now abandoned, which is a
continuation of U.S. patent application Ser. No. 12/749,334 filed
Mar. 29, 2010, now U.S. Pat. No. 8,198,581, which is a continuation
of U.S. patent application Ser. No. 10/587,478, filed on Sep. 4,
2008, now U.S. Pat. No. 7,714,283, which is a national stage entry
of PCT application no. PCT/GB2006/002028, filed Jun. 5, 2006, which
claims the priority benefit under 35 USC .sctn.119 to British
patent application serial no. 0511375.8, filed Jun. 3, 2005, now GB
patent no. 2434484, entitled "Electrostatic Trap", which
applications are incorporated herein by reference in their
entireties.
FIELD OF THE INVENTION
[0002] This invention relates to improvements in an electrostatic
trap (EST), that is, a mass analyser of the type where ions
injected into it undergo multiple reflections within a field that
is substantially electrostatic during ion detection, i.e., any time
dependent fields are relatively small. It relates in particular but
not exclusively to improvements in the Orbitrap mass analyser first
described in U.S. Pat. No. 5,886,346.
BACKGROUND OF THE INVENTION
[0003] Electrostatic traps (ESTs) are a class of ion optical
devices where moving ions experience multiple reflections in
substantially electrostatic fields. Unlike in RF fields, trapping
in electrostatic traps is possible only for moving ions. To ensure
this movement takes place and also to maintain conservation of
energy, a high vacuum is required so that the loss of ion energy
over a data acquisition time Tm is negligible.
[0004] There are three main classes of EST: linear, where ions
change their direction of motion along one of the coordinates of
the trap; circular, where ions experience multiple deflections
without turning points; and orbital, where both types of motion are
present. The so-called Orbitrap mass analyser is a specific type of
EST that falls into the latter category of ESTs identified above.
The Orbitrap is described in detail in U.S. Pat. No. 5,886,346.
Briefly, ions from an ion source are injected into a measurement
cavity defined between inner and outer shaped electrodes. The outer
electrode is split into two parts by a circumferential gap which
allows ion injection into the measurement cavity. As bunches of
trapped ions pass a detector (which, in the preferred embodiment is
formed by one of the two outer electrode parts), they induce an
image current in that detector which is amplified.
[0005] The inner and outer shaped electrodes, when energized,
produce a hyper-logarithmic field in the cavity to allow trapping
of injected ions using an electrostatic field. The potential
distribution U(r,z) of the hyper-logarithmic field is of the
form
U ( r , z ) = k 2 [ z 2 - r 2 2 ] + k 2 ( R m ) 2 ln [ r R m ] + C
( 1 ) ##EQU00001##
where r and z are cylindrical coordinates and z=0 is the plane of
symmetry of the field) C is a constant, k is the field curvature
and R.sub.m>0 is the characteristic radius.
[0006] In this field, the motion of ions with mass m and charge q
along the axis z is described as a simple harmonic oscillator with
an exact solution for q,k>0:
z(t)=A.sub.zcos(.omega..sub.0t+.theta.) (2)
where
.omega. 0 = qk m ( 3 ) ##EQU00002##
and T.sub.0 thus defines the frequency of axial oscillations in
radians per second, and A.sub.z and 2 are the amplitude and phase
of axial oscillations, respectively.
[0007] Whilst the foregoing discusses the theoretical situation, in
which the electrodes are of ideal hyper-logarithmic shape, in
reality there is a limit to the accuracy with which any practical
construction can approximate that ideal geometry. As discussed in
"Interfacing the Orbitrap Mass Analyser to an Electrospray Ion
Source", by Hardman et al, Analytical Chemistry Vo. 75, No. 7,
April 2003, any divergence from the ideal electrode geometry,
and/or inclusion of electrical perturbations, will result in a
perturbation to the ideal field which in turn will transform the
harmonic axial oscillations of the ideal field into non-linear
oscillations. This in turn may result in a reduction in mass
accuracy, peak shape and height, and so forth.
[0008] The present invention, in general terms, seeks to address
problems arising from the non-ideal nature of a real electrostatic
trap.
SUMMARY OF THE INVENTION
[0009] Against this background, aspects of the present invention
provide for an electrostatic ion trap in which deliberate
non-linearities or perturbations are introduced to the field so as
to control or constrain the rate of phase separation of ions within
a given bunch (of single m/z). In particular, the present invention
provides, in a first aspect, an electrostatic ion trap for a mass
spectrometer, comprising an electrode arrangement defining an ion
trapping volume, the electrode arrangement being arranged to
generate a trapping field defined by a potential
U'(r,.phi.,z)=U(r,.phi.,z)+W, where U(r,.phi.,z) is an ideal
potential which traps ions in the Z-direction of the trapping
volume so that they undergo substantially isochronous oscillations
and where W is a perturbation to that ideal potential U(r,.phi.,z),
wherein the geometry of the electrode arrangement generally follows
one or more lines of equipotential of the ideal potential
U(r,.phi.,z) but wherein at least a part of the electrode
arrangement deviates to a degree from that ideal potential
U(r,.phi.,z) so as to introduce the perturbation W into the said
trapping field, the degree of deviation from the ideal potential
U(r, .phi.,z) being sufficient to result in the relative phases of
the ions in the trap shifting over time such that at least some of
the trapped ions have an absolute phase spread of more than zero
but less than about 2.pi. radians over an ion detection period
T.sub.m.
[0010] According to a second aspect of the present invention, there
is provided an electrostatic ion trap for a mass spectrometer
comprising an electrode arrangement defining an ion trapping
volume, the electrode arrangement being arranged to generate a
trapping field defined by a potential U(r,.phi.,z) where
U(r,.phi.,z) is a potential which traps ions in the Z-direction of
the trapping volume so that they undergo substantially isochronous
oscillations, wherein the trap further comprises field perturbation
means to introduce a perturbation W to the potential U(r,.phi.,z)
so as to enforce a relative shift in the phases of the ions over
time such that at least some of the trapped ions have an absolute
phase spread of more than zero but less than about 2.pi. radians
over an ion detection period T.sub.m.
[0011] The specific description provides a detailed theoretical
analysis of the non-ideal electrostatic trap and the manner in
which perturbations W affect the overall performance of the mass
analyser. In general terms, however, it may be noted that there are
a very large number of trap parameters which affect the mass
analysis to varying degrees, including the degree to which the
field generation means approximates the ideal electric field, the
accuracy of various dimensions of the trap both in absolute terms
and relative to other components of the trap, the accuracy and
stability of any voltages applied to generate the field, and so
forth. Nevertheless, in broad terms these may be classified into
geometric distortions, such as "stretching" of the shape, shifting
of the spatial location of the electrodes relative to an
equipotential of the ideal field U(r,.phi.,z), oversizing or
undersizing the electrodes in one or more dimensions etc, and
applied distortions such as voltages applied to the trapping and/or
to additional distortion electrodes (e.g., end cap electrodes), or
applied magnetic fields, etc. Of course, whilst it is possible to
create the appropriate perturbation W using only one of these
(geometric or applied distortion), a suitable perturbation could of
course be created using a combination of both a geometric and an
applied distortion.
[0012] In terms of the effect upon the trapped ions, the non-ideal
nature of the trap results in one of two general situations. In the
ideal trap, the oscillations in the axial (Z) direction have a
frequency .omega..sub.0 that is independent of amplitude (apart
from a small, asymptotic shift due to space charge effects,
regarding which, see later). For a non-ideal trap, and assuming
that W, the perturbation, is a function of z (at least), the
oscillations in the z direction of ions are no longer independent
of amplitude. Instead, the ions either spread out (separate) in
phase over time or compress (bunch) together in phase. In the case
of phase bunching, this results in various undesirable artefacts
such as the so-called "isotope effect" (explained below), poor mass
accuracy, split peaks, poor quantitation (i.e. a distortion of the
relation between measured and real intensities of peaks) any one of
which may be fatal to the analytical performance of the trap. In
the case of phase separation, the spread of phases will continue to
increase with time. Once the phase spread exceeds .pi. radians,
ions start to move with opposite phases, resulting in compensating
image currents that progressively reduce the overall signal.
[0013] If the phase spreading occurs rapidly (relative to a
measurement time T.sub.m), then the desirable part of the signal is
essentially lost whilst the signal resulting from the phase bunched
ions is analytically poor or useless. The present invention in a
first aspect provides for a trap with parameters optimized so as to
constrain the rate of increase in phase spread. It is likely that a
real trap will have parameters that result in a perturbation to the
ideal field W which cause some phase spreading. However, if the
phase spreading is constrained so as to keep it below about 2.pi.
radians, for a time period commensurate with a trap measurement
period T.sub.m, then non-bunched ions will be detected without
degradation in analytical performance.
[0014] An alternative way of looking at this is to consider the
rate of decay of the `transient` detected by the detection means.
Typically, such a transient is generated by measuring the image
current induced in the detection means by ions in the trap. A trap
in which there is a rapid decay in the amplitude of the transient,
in the time domain, exhibits a poor analytical performance, and in
particular the mass accuracy tends to be poor in the Fourier
transformed signal.
[0015] Thus in accordance with a third aspect of the present
invention, there is provided an ion trap for a mass spectrometer,
comprising: electric field generation means to produce an electric
field within which the ions may be trapped; and detection means to
detect ions according to their mass to charge ratio; wherein the
electric field generation means is arranged to produce an electric
trapping field which traps ions so that they describe oscillatory
motion in which the period of oscillations is dependent upon the
amplitude of oscillations thereof, so as to cause a shift in the
relative phase of ions in the trap over time, wherein the detection
means is arranged to generate a time domain transient from the ions
in the trap, the transient containing information on those ions,
and further wherein the parameters of the trapping field are
arranged such that the detected transient decays from a maximum
amplitude to no less than a) 1%; b) 5%; c) 10%; d) 30%; e) 50% over
an ion detection time T.sub.m.
[0016] In yet another aspect of the invention there is provided an
electrostatic ion trap for a mass spectrometer comprising: electric
field generation means to produce an electric field within which
the ions may be trapped; and detection means to detect ions
according to their mass to charge ratio, wherein the electric field
generation means is arranged to produce an electric field of the
form, in cylindrical coordinates:
U ( r , .phi. , z ) = k 2 [ z 2 - r 2 2 ] + k 2 ( R m ) 2 ln [ r R
m ] + W ( r , .phi. , z ) ##EQU00003##
[0017] where U is the field potential at a location r,.phi.,z; k is
the field curvature; R.sub.m>0 is the characteristic radius, and
W(r,.phi.,z) is a field perturbation, and further wherein W is a
function of r and/or .phi. but not z, or wherein W is a function of
at least z but wherein, in that case, the field perturbation W
causes the period of oscillation of at least some of the ions along
the z axis of the trap to increase with the increase in the period
of oscillation in that z direction.
[0018] Various features of the trap have been ascertained through
experiment to result in a perturbation that causes phase bunching
to dominate, with the peak from non-bunched ion packets being lost
because of a rapid growth in phase shift. Preferred features of the
present invention propose controlled distortions to the trap
geometry, configuration and/or applied voltages so as to constrain
the rate of growth of non-bunched ion packets so that the phase
shift does not exceed about 2.pi. radians over the time scale of
ion measurement.
[0019] In accordance with a further aspect of the present invention
there is provided an electrostatic ion trap for a mass spectrometer
comprising: electric field generation means to produce an electric
field within which the ions may be trapped; and detection means to
detect ions according to their mass to charge ratio; wherein the
electric field generation means is arranged to produce an electric
trapping field which traps ions so that they describe oscillatory
motion in which the period of oscillations is dependent upon the
amplitude of oscillations thereof, so as to cause a shift in the
relative phase of ions in the trap over time, and further wherein
the parameters of the trapping field are arranged such that the
spread of phases of at least some of the ions in the trap to be
detected is greater than zero but less than about 2.pi. radians
over an ion detection time T.sub.m.
[0020] The invention also extends to a method of trapping ions in
an electrostatic trap having at least one trapping electrode,
comprising: applying a substantially electrostatic trapping
potential to the or each trapping electrode, so as to generate an
electrostatic trapping field within the trap, for trapping ions of
a mass to charge ratio m/q in a volume V such that they undergo
multiple reflections along at least a first axis z; and applying a
distortion to the geometry of the trap, and/or to the trapping
potential applied to the or each trapping electrode, so as to cause
a perturbation in the electrostatic trapping field which results in
at least some of the ions of mass to charge ratio m/q to undergo a
separation in phase of no more than about 2.pi. radians over a
measurement time period T.sub.m. Preferably, such separation should
be positive.
[0021] The invention also extends to a method of trapping ions in
an electrostatic trap having at least one trapping electrode,
comprising: applying a substantially electrostatic trapping
potential to the or each electrode, so as to generate an
electrostatic trapping field within the trap, for trapping ions in
a volume V such that they undergo multiple reflections, along at
least a first axis z, with a period of oscillation .tau. increasing
with increasing amplitude of oscillation A.sub.z of ions trapped in
the field over the volume V.
[0022] In still a further aspect of the invention, there is
provided a method of determining the acceptability or otherwise of
an electrostatic trap, comprising supplying a plurality of ions to
the trap; detecting at least some of the ions in the trap;
generating a mass spectrum therefrom; and either (a) ascertaining
whether or not the peaks in that mass spectrum are split, split
peaks being indicative of a poorly performing trap, and/or (b)
determining the relative abundances of isotopes of a known ion in
the mass spectrum, the degree to which these relative abundances
correspond with predicted (theoretical or naturally occurring)
abundances being indicative of the acceptability of the trap.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] The invention may be put into practice in a number of ways
and some specific embodiments will now be described by way of
example only and with reference to the accompanying Figures in
which:
[0024] FIG. 1 shows a schematic arrangement of a mass spectrometer
including an electrostatic trap and an external storage device;
[0025] FIG. 2 shows plots of the dependence of the amplitude of
oscillation on the period of oscillation in an ideal and a
non-ideal electrostatic trap;
[0026] FIG. 3 shows the change in relative phase of ions in the
electrostatic trap as a function of time t, in the presence of
various perturbing factors;
[0027] FIG. 4 shows a side sectional view of an electrostatic trap
in accordance with a first embodiment of the present invention;
[0028] FIG. 5 shows a side sectional view of an electrostatic trap
in accordance with a second embodiment of the present
invention;
[0029] FIG. 6 shows a side sectional view of an electrostatic trap
in accordance with a third embodiment of the present invention;
[0030] FIG. 7 shows a side sectional view of an electrostatic trap
in accordance with a fourth embodiment of the present
invention;
[0031] FIGS. 8a-8d show mass spectra from a first sample at around
m/z=195, with increasing degrees of non-linearity introduced into
the electrostatic field such that increasingly rapid phase
separation occurs;
[0032] FIGS. 9a-9d show mass spectra from a second sample at around
m/z=524, with increasing degrees of non-linearity introduced into
the electrostatic field such that increasingly rapid phase
separation occurs;
[0033] FIG. 10a shows a transient produced from an EST with
optimised parameters, resulting in a gradual spread of phases and a
gradual decay in the transient; and
[0034] FIG. 10b shows a transient produced from an EST with poor
parameters, resulting in a rapid spread of phases and a rapid
initial decrease in the magnitude of the transient.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0035] Referring first to FIG. 1, a schematic arrangement of a mass
spectrometer including an electrostatic trap and an external
storage device is shown. The arrangement of FIG. 1 is described in
detail in commonly assigned WO-A-02/078046 and will not be
described in detail here. A brief description of FIG. 1 is,
however, included in order better to understand the use and purpose
of the electrostatic trap to which the present invention
relates.
[0036] As seen in FIG. 1, the mass spectrometer 10 includes a
continuous or pulsed ion source 20 which generates gas-phase ions.
These pass through an ion source block 30 into an RF transmission
device 40 which cools ions. The cooled ions then enter a linear ion
trap acting as a mass filter 50 which extracts only those ions
within a window of mass charge ratios of interest. Ions within the
mass range of interest then proceed via a transfer octapole device
55 into a curved trap 60 which stores ions in a trapping volume
through application of an RF potential to a set of rods (typically,
quadrupole, hexapole or octapole).
[0037] As explained in more detail in the above-mentioned
WO-A-02/078046, ions are held in the curved trap 60 in a potential
well, the bottom of which may be located adjacent to an exit
electrode thereof. Ions are ejected orthogonally out of the curved
trap 60 into a deflection lens arrangement 70 by applying a DC
pulse to the exit electrode of the curved trap 60. Ions pass
through the deflection lens arrangement 70 and into an
electrostatic trap 80. In FIG. 1, the electrostatic trap 80 is the
so-called "Orbitrap" type, which contains a split outer electrode
85, and an inner electrode 90. Downstream of the Orbitrap 80 is an
optional secondary electron multiplier (not shown in FIG. 1), on
the optical axis of the ion beam.
[0038] In use, a voltage pulse is applied to the exit electrode of
the curved trap 60 so as to release trapped ions in an orthogonal
direction. The magnitude of the pulse is preferably adjusted to
meet various criteria as set out in WO-A-02/078046 so that ions
exiting the curved trap 60 and passing through the deflection lens
arrangement 70 focus in time of flight. The purpose of this is to
cause ions to arrive at the entrance to the Orbitrap as a
convolution of short, energetic packets of similar mass to charge
ratio. Such packets are ideally suited to an electrostatic trap
which, as will be explained below, requires coherency of ion
packets for detection to take place.
[0039] The ions entering the Orbitrap 80 as coherent bunches are
squeezed towards the central electrode 90. The ions are then
trapped in an electrostatic field such that they move in three
dimensions within the trap and are captured therein. As is
explained in more detail in our commonly assigned U.S. Pat. No.
5,886,346, the outer electrodes of the Orbitrap 80 act to detect an
image current of the ions as they pass in coherent bunches. The
output of the ion detection system (the image current) is a
"transient" in the time domain which is converted to the frequency
domain and from there to a mass spectrum using a fast Fourier
transform (FFT).
[0040] Having described the mode of operation of the Orbitrap 80
and its typical use within a mass spectrometer arrangement 10, a
theoretical analysis of the trapping of ions within the Orbitrap 80
will now be provided, in order to gain a better understanding of
the present invention.
Motion in an Ideal Field
[0041] As explained in U.S. Pat. No. 5,886,346, the ideal form of
electrostatic field within the Orbitrap 80 has a potential
distribution U(r,z), as defined in Equation (1) of the introduction
above. Note that, in Equation (1), the parameter C is a constant.
In this field, the motion of ions with mass m and charge q along
the axis z is described as a simple harmonic oscillator with an
exact solution defined in Equation (2) above, with .omega..sub.0=
{square root over ((qk/m))}, see Equation 3 above. In other words,
the period of oscillation .tau.(=2.pi./.omega..sub.0) in that z
direction is independent of the amplitude of oscillation of ions in
the z direction, A.sub.z.
Motion in a Perturbed Field: 2D Perturbation
[0042] In constructing a real electrostatic trap, the field defined
by Equation (1) can only be approximated due to finite
tolerances.
[0043] In cylindrical coordinates (r,.phi.,z), the potential
distribution U can be written, generally, as:
U ( r , .phi. , z ) , = k 2 ( z 2 - r 2 2 ) + k 2 ( R m ) 2 ln [ r
R m ] + W ( r , .phi. , z ) . ( 4 ) ##EQU00004##
[0044] Here, the parameters of the equation are as defined in
connection with Equation (1), save that the constant C is replaced
by a field perturbation W which is, in its most general form,
three-dimensional.
[0045] If we consider the situation where W does not depend on z,
and also satisfies the Laplace equation given by Equation (5)
below:
.DELTA.W(r,.phi.)=0 (5)
[0046] It may be shown that the motion of ions in the z direction
remains defined by Equations (2) and (3) above. In particular, the
period of oscillation .tau. (=2.pi./.omega..sub.0) remains
independent on the amplitude of oscillation A.sub.z in the z
direction. The general solution to Equation (5), in (xy)
coordinates, may be written as
U ( x , y ) = - k 4 [ x 2 - y 2 ] a + [ A r m + B r m ] cos { m cos
- 1 ( x r ) + .alpha. } + b ln ( r D ) + E exp ( F x ) cos ( F y +
.beta. ) + G exp ( H y ) cos ( H x + .gamma. ) ( 6 )
##EQU00005##
where r= {square root over ((x.sup.2+y.sup.2))}, .alpha., .beta.,
.gamma., .alpha., A, B, D, E, F, G, H are arbitrary constants
(D>0), and j is an integer. It should be noted that Equation (6)
is general enough to remove completely any or all of the terms in
Equation (1) that depend upon r, and replace them with other terms,
including expressions in other coordinate systems (such as
elliptic, hyperbolic, etc. systems of coordinates). However, such
great deviations from axial symmetry are rarely advantageous in
practice. The construction of an electrostatic trap is, in other
words, preferably such that the perturbation W remains small. For
example, matching elliptical deformation of both the inner and the
outer electrodes of the Orbitrap, or parallel shifting of the inner
electrode relative to the outer electrode along the x- or
y-coordinate, will have no influence on Equations (2) and (3) (such
that the period of oscillation .tau. remain independent of the
amplitude of axial oscillations), whilst the tolerance requirements
on such deformations for the construction of a trap which operates
within acceptable boundaries are less strict. Motion in a Perturbed
Field: Problems with 3D Perturbations
[0047] The primary difficulties with a real electrostatic trap
arise in the case where the perturbation W does depend on z (either
with or without an additional dependence upon r and/or .phi.). In
this case, Equations (2) and (3) are no longer exactly true and the
period of oscillation .tau. becomes a function of the amplitude of
oscillation A.sub.z. The vast majority of manufacturing
imperfections, to be discussed in further detail below, result in a
perturbation W that has a dependence upon z at least (and,
normally, also cross-terms r.sup.lz.sup.m cos.sup.n(.phi.), where
l, j, n are integers). The effect itself is very complex. However,
it is possible to obtain a useful and meaningful generalisation by
considering two simple but contrasting situations.
[0048] Referring to FIG. 2, some plots of the dependence of the
period of oscillation .tau. upon the amplitude of oscillation of
ions in the z direction are shown. The dotted line 200 represents
the ideal situation where there is no perturbation (that is, the
situation of Equation (1) or, alternatively, where the perturbation
is not dependent upon z (as described in "Motion in a Perturbed
Field: 2D Perturbation" above). The period of oscillation of ions
in the electrostatic trap remains constant, for a given mass to
charge ratio, regardless of the amplitude of those
oscillations.
[0049] Where the electrostatic field is slightly non-linear
(Equation (4)) and the perturbation W is dependent upon z, the
period of oscillation .tau. starts to depend upon A.sub.z. Line 220
in FIG. 2 illustrates, simplistically, the case where higher
amplitudes result in shorter periods of oscillation T. Ions in the
beam are spread over a range of amplitudes .DELTA.z and have a
spread of initial phases .DELTA..theta..sub.z. It will of course be
understood that the real dependence of the period of oscillation
.tau. upon amplitude of oscillation A.sub.z is most unlikely to be
linear for all possible A.sub.z, as line 220 suggests, but showing
a linear, monotonically decreasing period of oscillation .tau. with
increasing A.sub.z permits more straightforward explanation. The
situation where the dependence of period upon amplitude does not
increase or decrease in a linear, monotonous fashion will be
explored below.
[0050] For ions in the ideal field of Equation (1), and in absence
of any collisions, the oscillation according to Equations (2) and
(3) without shift of parameters will result in a fixed phase spread
.DELTA..theta. over time t. This is shown as dotted line 300 in
FIG. 3.
[0051] Where the perturbation results in a slightly non-linear
electric field, due to the perturbed potential distribution defined
by equation (4), and that perturbation has a dependence upon z, the
ions will still move in accordance with Equations (2) and (3).
However, ions will now have a phase .theta. which changes with time
t. In the case of a dependence of period .tau. on amplitude A.sub.z
that is as shown by line 220 in FIG. 2 (.tau. decreases with
increasing A.sub.z), the spread of phases will increase with time.
This is because ions with a higher A.sub.z will move faster,
relatively speaking, and ions with lower A.sub.z will move
relatively slower. The increase in the spread of phases as a
consequence is shown by dotted line 310 in FIG. 3.
[0052] At the point where the phase spread exceeds .pi. radians,
ions start to move with opposite phases. This in turn compensates
image currents of each other which progressively reduces the
overall signal.
[0053] There is a minimum detection period within the Orbitrap. The
longer the detection period, the higher the resolution. On the
other hand, extended measurement periods result in a phase spread
shift that exceeds .pi. radians. Therefore, it may be seen that a
first restriction upon the manufacture of a real electrostatic trap
is that any perturbation introduced should result in a net change
in relative phase of no more than about 2.pi. radians, preferably
no more than .pi. radians, over a sufficiently long measurement
period T.sub.m.
[0054] In fact, in a real trap, the increase in phase spread over
time is generally not simply a result of a slightly non-linear
field (due to a perturbation of the potential, W). When the number
of ions in a beam is increased beyond a certain level (typically,
beyond 10,000 to 100,000 ions), ion-ion interactions start to
affect ion motion, as a consequence of space charge. In the ideal
field (1), this results in a spreading of an ion beam that slows
down with time, as the ion packets becomes large enough that the
distance between ions reaches a high level. This small,
time-dependent drift of phase .theta., which is a consequence of
space charge and occurs even in the absence of a perturbation of
the potential, is a known phenomenon and is shown schematically as
line 320 in FIG. 3. It will be seen the line 320 asymptotically
approaches a line with a non-zero slope.
[0055] In the case of a non-linear electric field, due to the
perturbed potential distribution described by equation (4), which
results in a period of oscillations .tau. that increases with
increasing amplitude A.sub.z (line 210 of FIG. 2), this small
time-dependent phase drift resulting from space charge effects is
still present. In this case, however, the space charge effects
represented by line 320 are associative with the increase in phase
resulting from the dependence of period on amplitude given by line
210 in FIG. 2 and shown as line 310 in FIG. 3. Adding lines 310 and
320 results in line 330 of FIG. 3. Thus it will be seen that, even
with the effects of space charge, the consequence of a perturbation
on the ideal field which results in a period of oscillations
decreasing with increasing amplitude A.sub.z is that the line 330
reaches the .pi. radian phase shift in less time. As explained
above, this means that, for a given construction of electrostatic
trap, the space charge effect merely reduces the maximum suitable
measurement period T.sub.m.
[0056] The consequences of a perturbation W resulting in a period
of oscillation .tau. that decreases with amplitude A.sub.z is more
problematic, however. Line 220 in FIG. 2 illustrates, again
schematically and for the purposes of example only, this situation.
Physically, the consequence of a dependence such as is shown in
line 220 of FIG. 2 is that ions are "bunched" together. The reason
for this is as follows. The small time-dependent drift of phase
.theta. resulting from space charge is still present. However, this
combines with the effect of the non-linear field which results in
the dependence of T on A.sub.z shown in line 220 of FIG. 2 to
produce a shift in phase illustrated by line 340 of FIG. 3.
[0057] One possible mechanism for this counter-intuitive behaviour
is as follows. Ions at the edge of the ion beam are pushed to
smaller or larger A.sub.z. For example, an ion on the right-hand
edge of the range of amplitudes A.sub.z of FIG. 2 is pushed by the
space charge effect of other ions to a larger A.sub.z, at the same
time lagging in phase .theta.. As a result of the dependence shown
by line 220, however, a larger amplitude A.sub.z corresponds to a
lower period of oscillation .tau. (and a higher frequency
.omega..sub.0) of oscillations, so that the ion is forced to catch
up in phase .theta. and return to the same phase as ions in the
middle of the beam.
[0058] Similarly, ions that are pushed to a smaller amplitude
A.sub.z and forward in phase .theta. become slower and also return
back to the same phase as ions in the middle of the beam. As a
result, rather than continuously increasing the ion beam phase
spread (as occurs in the other situation resulting in line 330
above), the ion beam stops increasing its phase spread. For certain
non-linearities, as shown by line 340, the phase spread may even
begin to decrease over time. Whilst at first glance this may appear
desirable, in fact it has a number of consequences which are at
best highly undesirable, and at worst can result in an unacceptably
poor performance of the electrostatic trap. For example, the peak
frequency will shift as a consequence of the curve 340, which in
turn affects the measured m/q. In some cases, for example when
non-linearity varies significantly over the cross-section of the
ion beam, the beam may even split into two or more sub-beams, each
with its own behavior. This will result, in turn, in split peaks
(shown in FIGS. 8d and 9d in particular, regarding which, see
below), poor mass accuracy, incorrect isotopic ratios (as an
intense ion beam decays more slowly than a less intense beam), poor
quantitation etc. Moreover, these effects may well be different for
differing mass to charge ratios, so that, even if a device can be
optimised to minimise phase bunching for a specific mass to charge
ratio, this may not improve (or may even make worse) the situation
with other mass to charge ratios.
[0059] In reality, the perturbation W will have a complex structure
such that different parts of the same ion beam, with the same mass
to charge ratio, may experience vastly different effects. For
example, one part of the beam could be self-bunched with one
average rate (d.theta./dt).sub.1, a second part of the beam may
experience rapid phase spreading (within time t<<T.sub.m),
with a third part of the beam self-bunched at a different rate
(d.theta./dt).sub.2. This will result in a split peak with a part
of the peak at a frequency .omega..sub.0+(d.theta./dt).sub.1 and
another part at a different frequency
.omega..sub.0+(d.theta./dt).sub.2. The second part of the beam,
which has experienced rapid phase expansion, will be greatly
suppressed, again as explained above. Even more complicated
scenarios can be envisaged and, rapidly, the mass accuracy of the
device can be fatally compromised.
[0060] The foregoing discussion leads to the following conclusions.
There is nothing that can be done from an electrostatic field point
of view to avoid the inevitable space charge effects which result
in a small drift in phase. It is also unrealistic to expect that
the parameters of the trap can, in manufacture, be kept to such a
tight tolerance that there is no perturbation to the ideal field
(1) at all. Thus, the most preferred realistic scenario is that the
parameters of the trap are optimised so that the electrostatic
field is approximately hyper-logarithmic and has a perturbation to
it W which is dependent on r and/or .phi. only. In this case, other
than the small time dependent phase shift resulting from space
charge, the phase shift of ions over time should be zero.
[0061] In the case where the perturbation W depends upon z as well
as, or instead of, r and/or .phi., it is desirable to ensure that
the trap parameters are optimised so that there is phase spreading,
rather than phase bunching, over time, and that the phase spreading
is at a sufficiently low rate that the time taken for the net phase
spread to exceed .pi. radians is greater than an acceptable
measurement time period T.sub.m. This is not to imply that there
can be no phase bunching at all, and indeed a small degree of phase
bunching even without any phase separation may produce an
acceptable performance, only that it is preferable that at least a
majority of non-bunched ions survive with a phase spread less than
2.pi. radians for the entire measurement period. The difficulties
that result from phase bunching become less and less pronounced as
the growth of .DELTA..theta. over the measurement time scale
T.sub.m decreases.
[0062] There are, of course, a large number of parameters that vary
in the construction of an electrostatic trap, however, a number of
particularly desirable optimisations have been identified. These
have been implemented and are described now with reference to FIGS.
4 to 7. Referring first to FIG. 4, a schematic side view of an
Orbitrap 80 is shown. The operation of the Orbitrap is as
previously described and as set out in detail in, for example, U.S.
Pat. No. 5,886,346. The Orbitrap 80 comprises an inner electrode 90
(shown in end section in FIG. 1) and split outer electrodes 400,
410. As may be seen in FIG. 4, the electrodes are shaped, so far as
is possible within manufacturing tolerances, to have the
hyper-logarithmic shape of Equation (1). Within the outer electrode
410 is a deflector 420. Ions are introduced into the trapping
volume defined between the inner electrode 90 and outer electrodes
400, 410 through a slot 425 between the outer electrodes 400,
410.
[0063] End cap electrodes 440, 450 contain ions within the trapping
volume. An image current is obtained using a differential amplifier
430 connected between the two outer electrodes 400, 410.
[0064] In one embodiment, the outer electrodes 400, 410 are
stretched in the axial (z) direction. Axial stretching of the outer
electrodes relative to the ideal shape improves mass accuracy over
a wide mass range for ions injected using electrodynamic squeezing
as described by Makarov in Analytical Chemistry Vol. 72 (2000)
pages 1156-1162. Moreover, the inner electrode 90 may be radially
compressed around its axis of symmetry in order to introduce a
perturbation that results in gradual phase spreading. Additionally
or alternatively, voltages may be applied to the end electrodes
440, 450.
[0065] Since the ions exhibit harmonic motion along the z-axis of
the trap, the ions exhibit turning points towards the extremities
of the trap (+/-z). At these points, the ions are moving relatively
slowly and thus experience the potential towards the trap
extremities (in the axial direction) for longer than they
experience the potential in the vicinity of the centre slot 425
(FIG. 5). The ions at these turning points are also relatively
close to the outer electrodes. The result of this is that the shape
of the trap in the vicinity of the turning points has a relatively
significant impact on the ions. On the other hand, these turning
points are axially inward of the outer extremities of the trap. In
consequence, the shape of the trap at its axial extremities
(outside of the turning points) has relatively limited effect upon
the ions, since it is only the far field of these regions that
affect the ions in the region of the turning points. In particular,
the shape of the trap over the last 10% of its length is largely
irrelevant.
[0066] As may be seen in FIG. 5, the ion injection slot 425 is
axially central. The ions pass this point at maximum velocity and
thus spend statistically less time there. They are also well spaced
from the outer electrodes at that point. Thus, whilst the shape of
the trap there has some impact on the ion trajectories, it is not
so critical as the shape of the trap at the turning points. On the
other hand the ion injection slot 420 in the embodiment of FIG. 4
is located away from the central (z) axis, and is generally in the
region of one of the ion turning points. Thus the shape of the trap
in the region of the slot 420 is relatively critical to trap
performance.
[0067] As a related issue, it transpires that there is no apparent
need to provide compensation (at the electrode extremities) for the
truncation of the electrodes relative to their ideal infinite
extent.
[0068] FIG. 5 shows an alternative arrangement to the embodiment of
FIG. 4, although it is to be understood that the modifications and
features of FIG. 5 are by no means mutually exclusive with those
applied to the arrangement of FIG. 4. Nevertheless, features common
to FIGS. 4 and 5 have been labelled with like reference
numerals.
[0069] In FIG. 5, a spacer electrode 460 is mounted between the
outer electrodes 410, 420 and a voltage may be applied to this. In
general terms, employing a spacer between the outer electrodes so
as to shift them apart may be desirable.
[0070] FIG. 6 shows still another embodiment. Here, the outer
electrodes 400, 410 are segmented into multiple sections 400',
400'', 410', 410''. In that case, bias voltages may be applied to
the segments. Each of the segment pairs may also be used for ion
detection in this mode, allowing detection at multiples of ion
frequency. For example, a triple frequency can be detected in the
arrangement of FIG. 6 without the loss of signal to noise ratio, if
the differential signal is collected between connected segment
pairs 400'-410', and 400''-410''. As another example, the signal
may be detected between 400' and 410'' (for example, with segment
400'' and segment 410' grounded or biased), providing strong third
harmonics of axial frequency, albeit at a lower signal to noise
ratio. An increase in the detection frequency provides a benefit of
higher resolving power within the limited detection time T.sub.m.
This is particularly useful for higher mass to charge ratio
ions.
[0071] Turning finally to FIG. 7, still a further embodiment of an
electrostatic trap 80 is shown. As with the arrangement of FIG. 4,
the Orbitrap 80 comprises a pair of outer electrodes 400, 410 with
a differential amplifier 430 connected across these. The outer
electrode 410 also includes a compensation electrode 420.
[0072] The inner electrode 90, however, is split into two segments
90', 90''. Bias voltages may be applied to the segments. In
addition to the segmentation, a spacer electrode 470 may also be
included, preferably on the axis of symmetry (z=0). Different
segments could, of course, also be employed for detection with or
without the outer electrodes.
[0073] Although a number of different embodiments have been shown,
it is to be understood that these are simply examples of
adaptations to the dimensions, shape, size, control and so forth of
the trap, to minimise the effect of perturbations that cause phase
bunching and to maintain perturbations which optimise (i.e.
minimise) the rate of increase of phase separation over the
measurement period T.sub.m. Any of the combinations described in
connection with FIGS. 4 to 7 may be combined. Other means may be
employed to produce multipole fields, that is, fields containing
terms proportional to z.sup.n, where n>2. Moreover, the Orbitrap
80 may be immersed in a magnetic field which provides mass
dependent correction of aberrations. This may be especially
effective for low mass to charge ratio ions that usually suffer the
greatest scattering during extraction from an external storage
device, an effect which is described in further detail in
WO-A-02/078046.
[0074] It is also to be appreciated that the voltage on the
deflection electrode 420 (FIGS. 4 and 7) should be chosen in such a
way that the deflection electrode itself contributes a minimal
non-linearity to the field. In general terms, the geometric
distortions described in connection with FIGS. 4 to 7 have a
magnitude of a few, to a few tens of, microns.
[0075] Empirically, some optimal ranges for geometric distortions
have been determined and are listed below. Once more, it is
stressed that these are experimentally observed observations that
result in a limitation in the phase spread and are in no way
intended to be limiting of the general inventive concept. In the
following list, the dimension D2 is (as indicated in FIG. 6) the
inner diameter of the outer electrodes 400, 410, at the axis of
symmetry (z=0). The dimension D1 is the outer diameter of the
central electrode 90, again the axis of symmetry (z=0).
[0076] (A) For present day machining technology, the optimal inner
diameter of the outer electrodes D2 is between 20 and 50 mm,
optionally 30 mm.+-.5 mm;
[0077] (B) In preference, D1<0.8D2, optionally 0.4D2.+-.0.1D2;
(so that the inner electrode diameter D1 is preferably 12 mm when
D2 is as in (A) above).
[0078] (C) The parameter R.sub.m in Equation (1) and Equation (4)
is preferably in the range 0.5D2<R.sub.m<2D2, and optionally
0.75D2.+-.0.2D2;
[0079] (D) The width of the entrance slot 425 (FIG. 4, for
example), in the z direction, should in preference lie in the range
0.01D2 to 0.07D2 and optionally between 0.02D2 and 0.03D2, and, in
the direction perpendicular to z (that is, in a direction looking
into the page when viewing FIG. 4, for example), should be less
than 0.2D2, optionally between 0.12D2 and 0.16D2;
[0080] (E) The overall inner length of the system should be greater
than twice (D2-D1), and most preferably greater than 1.4 times
D2;
[0081] (F) The accuracy of the shape of the outer electrodes,
relative to the hyper-logarithmic form of Equation (1) should be
better than 5.times.10.sup.-4D2, and optionally better than
5.times.10.sup.-5D2; where the inner diameter of the outer
electrode is 30 mm, the total deviation is preferably 7:m or
better. It has been found that the trap performance is better when
the diameter of the outer electrodes is either nominally ideal or
is slightly oversized (i.e. not undersized). By contrast the
performance is enhanced when the central electrode is undersized
(that is, too thin) by a few micrometers when the central electrode
is of nominal maximum diameter 6 mm, a slightly (-4:m to -8:m)
thinner electrode improves trap performance. Central electrodes of
the correct nominal diameter or larger appear to result in a trap
of reduced performance. One feasible explanation for this is that a
slightly undersized central electrode introduces a negative high
powered term (such as a fourth or higher power term) in the
potential distribution parallel to the z-axis at a given diameter.
The resultant slightly "flattened" potential, provided not too
large, exerts a sufficient but not excessive force on the ions to
prevent the unwanted "self-organization" of ions described above.
In other words, the -x.sup.4 or other high order term introduced by
a slightly undersized central electrode appears to promote a slow
phase spread. This is a desirable situation--the phase does spread
(which prevents bunching) but not too fast to prevent ion detection
in an acceptable time scale.
[0082] (G) The gap between the outer electrodes should be less than
0.005D2, in preference, and optionally around 0.001D2. It has
however been ascertained that the axial gap between the outer
electrodes may be 2-4:m too large without destroying the trap
performance;
[0083] (I) The additional axial stretching of the outer electrodes
relative to the ideal shape should be preferably in the range of 0
to 10.sup.-3D2, and optionally less than 0.0003D2;
[0084] (J) The degree of allowed tilt of the central electrode
should be less than 1% of D2 and preferably less than 0.1% D2;
[0085] (K) The allowed misalignment of the outer electrodes should
be less than 0.003D2 and preferably less than 0.0003D2;
[0086] (L) The allowed systematic mismatch between outer electrodes
should be less than 0.001D2 and preferably less than
5.times.10.sup.-5D2. In general, the mirror symmetry between the
injection and detection sides of the Orbitrap appears to be very
important. Typically, it is desirable that the maximum diameters of
the left and right outer electrodes match each other to within
around 0.005% which corresponds to 1-2:m in a 30 mm diameter trap;
and
[0087] (M) The allowed surface finish should be better than
2.times.10.sup.-4D2 and optionally less than 3.times.10.sup.-5
times D2. However, small, random variations in surface smoothness
seem to have a beneficial effect. In other words, random surface
defects appear to provide improved performance whereas long range
(systematic) variations reduce performance.
[0088] It will be apparent from the foregoing (and with reference
to the examples described below in connection with FIGS. 8, 9 and
10) that the different parameters, do not generally result in a
`perfect` or `useless` trap but instead combine with one another in
a complicated manner to present a trap that lies in a range between
these two extremes. Observations nevertheless confirm that, where
the parameters are within the ranges specified below, acceptable
traps are produced; where the parameters are optimised to the
magnitudes listed, currently good traps with correct peak shapes
and positions are produced. Moreover, of the above, items (D), (E),
(F), (G) and (H) appear to contribute most markedly to a degrading
perturbation which forces dominance of phase bunching. Thus
particular care should be taken in construction, to minimise the
amplitudes or dimensions within the preferred ranges.
[0089] The foregoing description has explained a feasible physical
basis for a degradation in the performance of a real electrostatic
trap, in terms of perturbations to the ideal electrostatic field
and the requirement that there should be at least a proportion of
the ions which are not phase-bunched but which do not
phase-separate too rapidly, if acceptable trap performance is to be
realised. By controlling the parameters of the trap, for example by
closely controlling the ranges of the parameters set out in (A) to
(M) above, the degree to which any real trap meets the criterion of
the present invention (minimising the rate of increase of phase
spread) can be determined directly. However, again empirically, a
number of indicators of likely trap performance (that is,
likelihood that the specific requirement regarding rate of increase
of phase spreading over the measurement period T.sub.m) exist.
[0090] Various elements have several isotopes which exist in nature
at a well known and defined ratio of relative abundances. For
example, carbon has two stable isotopes, .sup.12C, .sup.13C which
exist in nature in the ratio of approximately 98.93% and 1.07%
respectively. By obtaining a mass spectrum of the carbon isotopes
using a candidate electrostatic trap, the measured relative
abundances of the isotopes can provide an indication of the likely
suitability of that candidate trap that is, the likelihood that it
will meet minimum performance requirement. The consequence of a
badly-performing trap, in which non-self-bunching signals decay
very quickly (over time t<<T.sub.m) results in only
self-bunched signals (such as in curve 340 of FIG. 3) surviving.
Although such self-bunched signals give the impression of
acceptability, since peaks in a mass spectrum are narrow and peak
intensity is good, the smaller isotopic peak for .sup.13C appears
much smaller than natural abundance ratios would predict. It may
also be split into two or more sub peaks.
[0091] As a rule of thumb, therefore, if a real trap indicates an
apparent natural abundance of .sup.13C of less than about 0.7%
(where its predicted abundance should be in the region of 1.07%),
the trap would typically be rejected.
[0092] FIGS. 8a-d and 9a-d show plots of ion abundance against m/z
(i.e., mass spectra) for m/z around 195 and m/z around 524,
respectively, with differing amounts of field perturbation. In
particular, FIG. 8a shows a zoom-in of mass spectrum at nominal
mass 195. FIG. 9a shows a mass spectrum with a main peak at nominal
mass 524 and two smaller peaks at nominal masses 525 and 526
indicative of the presence of two isotopes. The label for each peak
lists m/z to 4 decimal places together with the resolving power of
the Orbitrap. The relative abundances of these two isotopic peaks
(normalized to the intensity of the main peak) are 26% and 4%
respectively, in the ideal limit.
[0093] FIGS. 8a and 9a are obtained from an Orbitrap that operates
with excellent parameters, that is, the rate of decay of the
transient (or, put another way, the rate of increase in phase
separation) is very slow. Here, peak resolution is limited by the
length of the stored transient (i.e. the measurement time T.sub.m),
which in FIGS. 8a and 9a is 0.76 seconds.
[0094] FIGS. 8b and 9b show mass spectra over the same ranges,
using the same ions, but with a slight non-linearity in the
electrostatic trapping field resulting in a discernable but
acceptable amount of phase spreading over the measurement time
T.sub.m. It will be noted in FIG. 8b that the main peak has
developed small wings on each side and that the measured peak
position is also shifted very slightly to a lower apparent m/z.
FIG. 9b also shows a very slight shift in the peak positions of the
main peak and the two isotopes, and also the relative abundances of
the isotopes are slightly different from those predicted.
Nevertheless, the peaks do show good shape and there is no peak
splitting.
[0095] Turning to FIGS. 8c and 9c, the mass spectra of an Orbitrap
with an unacceptably rapid phase expansion are shown, again for the
same ions as were employed in respect of FIGS. 8a, 8b, 9a and 9b
respectively. In FIG. 8a, the main peak is seen to be badly
suppressed (abundance less than 40% of the `true` abundance
illustrated in FIG. 8a) and with a larger number of adjacent peaks
which alter the true shape of the peak as well. FIG. 9c illustrates
the problems of rapid phase expansion (leaving just phase bunched
ions to be detected within a short amount of time, relative to the
total measurement time T.sub.m) as well. The main peak is
suppressed (although in FIG. 9c its intensity has been renormalized
to 100%) and the two isotopes show a much higher relative abundance
than they should (around 37% and 7% respectively, compared with
theoretical values of 26% and 4.5%). Inset into FIG. 9c is a zoomed
part of the spectrum around the main peak, contrary to the correct
appearance (that is, the peak shape of FIGS. 9a and 9b).
[0096] Finally, for completeness, FIGS. 8d and 9d show mass spectra
where a very large non-linearity exists or is added to the trap so
that any ions that are not phase bunched become undetectable within
a very short timescale (<<T.sub.m). In FIG. 8a the poor peak
shape is apparent--the narrow `spike` is a result of the phase
bunched ions and the smeared signal either side of that spike is a
result of the rapidly decaying phase spreading signal. The mass
spectrum of FIG. 9d demonstrates similar problems with the main
peak (a sharp spike resulting from phase bunched ions together with
a wide spread of minor peaks surrounding the main peak). Moreover,
the smaller isotopic peaks are also severely split (into a `spike`
and a spread of side bands) due to the phase bunched and rapidly
phase spreading ions respectively. The relative magnitudes of the
main and isotope peaks are also nowhere near the theoretical
values.
[0097] FIGS. 10a and 10b show transients (in the time domain) from
traps with rapidly and slowly increasing phase spreads,
respectively. It will be seen in FIG. 10a how the transient clearly
contains a rapidly decaying component (over approximately 200 msec)
and a slower decaying component (beyond 200 msec or so). This is
what results in the split peaks of FIGS. 9c and 9d, for example.
FIG. 10b, by contrast, shows a transient with a much more gradual
decay, even over 3 seconds (note the difference in scales on the
`x` axis, between FIGS. 10 and 10b). The transient of FIG. 10b,
once transformed into a mass spectrum, shows good mass accuracy,
peak shape and so forth, as illustrated in FIGS. 8a, 8b, 9a and
9b.
[0098] Another indicator of poor trap parameters is the presence of
an unusual non-linearity in the mass calibration. For example, if a
non-monotonous dependence is noted in the mass range, rather than a
linear function, it is generally concluded that the trap parameters
will not meet the requirement for the maximum rate of phase
spreading. Good Orbitraps tend to have a specific dependence of
mass deviation on ion injection energy: from 0 to 40 ppm per 150V
injection energy increase appears to be indicative of a functional
trap. Those traps exhibiting a negative slope (of about -5 to -10
ppm or more) do not generally work. To an extent this can be
mitigated (compensated) by the use of a larger spacer electrode 460
(FIG. 5), which results in the outer electrodes 410, 420 being
moved outwards, which in turn weakens the field at the trap
edges.
[0099] Finally, as explained above, the presence of split peaks,
resulting from the complex structure of the perturbation W,
normally provides a good clue that the performance of the trap in
general will not be acceptable.
[0100] To optimise the stability of the construction of an
electrostatic trap, having optimised the parameters themselves such
as in accordance with (A) to (M) above, it is preferable to use
temperature invariant materials in the design, such as Invar.TM.
for the trap itself, and quartz or glass for insulation. In
addition, high or ultra-high vacuum should be maintained within the
volume traversed by the ions.
[0101] It is of course to be understood that the invention is not
limited to the various embodiments of Orbitrap described above, and
that various modifications may be contemplated. For example, as
described in our copending application no GB0513047.1, the contents
of which are incorporated by reference in their entirety, the
Orbitrap electrodes may be formed from a series of rings rather
than one or more solid electrodes. In that case, in order to
introduce the desirable perturbation W to the ideal
hyperlogarithmic electrostatic potential U(r,.phi.,z), the rings
can be manufactured to have a shape that conforms to an
equipotential of the perturbed field U'(r,.phi.,z). On the other
hand, it may be preferable as well or instead to separate or
compress some or all of the rings relative to one another in the
axial (z) direction to create the same effects as are listed in
(A)-(M) above. For example, spreading the outer electrode rings
relative to the ideal equipotential mimics the desirable
"flattened" shape discussed in (F) above. Compressing the inner
rings together likewise mimics the smaller diameter inner electrode
arrangement that is beneficial.
[0102] Indeed, the invention is not limited just to the Orbitrap.
The ideas may equally be applied to other forms of EST including a
multi-reflection system with either an open geometry (wherein the
ion trajectories are not overlapping on themselves after multiple
reflections) or a closed geometry (wherein the ion trajectories
repetitively pass through substantially the same point). Mass
analysis may be based on frequency determination by image current
detection or on time-of-flight separation (e.g. using secondary
electron multipliers for detection). In the latter case, it will of
course be apparent that a phase spread of 2.pi. radians corresponds
with a spread of time-of-flights of ions of one period of
reflection. Various examples of ESTs to which the invention may be
applied are described in the following non limiting list: U.S. Pat.
No. 6,013,913, U.S. Pat. No. 6,888,130, US-A-2005-0151076,
US-A-2005-0077462, WO-A-05/001878, US-A-2005/0103992, U.S. Pat. No.
6,300,625, WO-A-02/103747 or GB-A-2,080,021.
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